How about a NV SRAM, such as the DS1220? 100 nsec,
24-pin JEDEC
pinout. Or an ST M48Z12, same-oh pinout and capacity?
These look like the best option.
Are you trying to replace something like an 82S191?
Indeed , I have a CPU with 40 bit wide microword (5x 82s191).
I would like to replace the instructions for read sector, write sector and set track.
Target is to use an IDE drive instead of the original cartridge drive.
Jos